1. Field of the Invention
The present invention is directed to the use of a switch element for switching in an electrical line system of a magnetic resonance tomography apparatus, and to a magnetic resonance tomography apparatus of the type having a radio-frequency coil, particularly a radio-frequency reception coil, and with a switch element for influencing the coil activity.
2. Description of the Prior Art
Magnetic resonance tomography systems of the above type are know, as are magnetic resonance tomography systems having a number of radio-frequency transmission coils and a number of radio-frequency reception coils with at least one switch element for multiplexing control signals or for demultiplexing reception signals.
In a magnetic resonance tomography apparatus, radio-frequency pulses are emitted into the examination region of a subject or of a patient under examination in the presence of a uniform, time-constant basic magnetic field. During this transmission phase, a radio-frequency transmission coil is fed by a power amplifier. The radio-frequency transmission coil transmits at the Larmor frequency of the hydrogen nuclei that is typical for the basic field. As a consequence of the application of the radio-frequency pulse, an echo signal that can be received by a radio-frequency reception coil acting as antenna arises at a later point in time when the radio-frequency transmission pulse has already been deactivated. A raw data matrix is compiled from a number of radio-frequency echo signals that have been acquired with successive variation of an applied, gradient magnetic field. An image of the region under examination in the patient or subject is generated from this raw data matrix by Fourier transformation.
The radio-frequency transformation coil and the radio-frequency reception coil can be the same radio-frequency coil. In this case, the supply line of the radio-frequency coil must be switched back and forth between the power amplifier and a pre-amplifier serving the purpose of amplifying incoming echo signals, given each change between the transmission phase and the reception phase.
A number of radio-frequency reception coils can be present, for example a permanently installed reception coil, a local coil fashioned as a volume coil and/or a local coil fashioned as a surface coil. A single electronic reception channel can be present for these reception coils, so that a demultiplexing of the reception signals generated by the various reception coils is necessary.
In the case of the radio-frequency reception coil be separate from the radio-frequency transmission coil, it is necessary to detune the radio-frequency reception coil during the transmission phase, since the radio-frequency reception coil is likewise tuned to the Larmor frequency during the reception phase, because the radio-frequency reception coil would otherwise pick up power from the radiated radio-frequency field with high efficiency. This must be avoided in order to protect the patient. A known radio-frequency reception coil therefore has a detuning circuit. A PIN diode is integrated therein that activates the detuning circuit in the through-connected state, so that the radio-frequency reception coil is deactivated, i.e. detuned. The radio-frequency reception coil then can no longer receive.
The PIN diode in the detuning circuit acts as a fast, active switch element. A PIN diode, however, can act as a good switch only with low ON resistance when it is integrated for this purpose in the existing resonant circuit of the detuning circuit. In the through-connected condition of the PIN diode, the resonant circuit of the detuning circuit, which is tuned to the resonant frequency of the magnetic resonance tomograph (Larmor frequency), acts as a blocking circuit that minimizes the current in the radio-frequency reception coil acting as antenna in the transmission phase.
Conventional resonant detuning circuits of this type have the disadvantage that all of the voltage up to 500 V induced by the radio-frequency reception coil drops across the detuning circuit, so that an extremely high current flows in the blocking circuit during the transmission phase. This currentxe2x80x94if only because of the magnetic field connected therewithxe2x80x94results in disturbances in the image acquisition in the nuclear magnetic resonance tomography apparatus.
An object of the present invention is to provide a switch element for employment in a magnetic resonance tomography apparatus that can be utilized in the frequency range of radio-frequency fields and that satisfies the switching times required in nuclear magnetic resonance tomography, and with which, in particular, the aforementioned disadvantages can be avoided or minimized.
A further object of the invention is to provide a magnetic resonance tomography apparatus having such a switch.
The object directed to a suitable switch element is inventively achieved by employing an electrostatic relay in an electrical line system of a magnetic resonance tomography apparatus.
The invention is based on the perception that an electrostatic relay is well suited for utilization in the presence of a strong magnetic field (constant magnetic basic field). An electrostatic relay operates largely free of current, so that interaction with the magnetic basic field does not occur. An electrostatic relay can be constructed without employing a coil whose magnetic field would be influenced by the basic field.
In contrast to a diode or a tube, that function as purely electronic switch elements, a relay is an electromechanical switch that can be electrically actuated but wherein the actual interrupt event occurs mechanically. The relay producing a completely galvanic separation. A relay can be implemented with an especially low ON resistance and with high dielectric strength in the radio-frequency range. Moreover, extremely short switching times can be achieved with the electrostatic relay. A PIN diode is a switch element that is voltage-stable at high frequencies and that would also be adequately fast for many purposes, however, this can only be utilized in a magnetic resonance apparatus by accepting the disadvantages that were set forth above.
An electrostatic relay can be utilized with particular advantages for employment in the uniform basic field of a magnetic resonance tomography apparatus preferably is arranged in the region of this basic field.
In a preferred version of the inventive method, a micro-mechanical relay is employed as relay, this being preferably at least partially manufactured with a material erosion technique, for example by etching, from a substrate, particularly a silicon substrate. Such a micro-mechanical relay can, for example, have a structural height in the range of 0.5 nm through approximately 1 mm that lies in the range of the thickness of typical silicon wafers. Moreover, the advantage is achieved that a micro-mechanical electrostatic relay can implement especially fast switching events, since the masses to be moved and, thus, the forces of inertia that occur are small. The distance over which the mass must be moved also is small.
German OS 32 07 920, corresponding to U.S. Pat. No. 4,480,162, discloses a micro-mechanical electrostatic relay for telephone systems that is suitable for the inventive employment. The inventively employed electrostatic relay has, in particular, a carrier part composed of a material having an electrically insulating effect that forms a carrier member and a structural unit with the carrier member and has an armature secured in articulated fashion to the carrier that carries at least one electrical contact. The teachings of German OS 32 07 920 and U.S. Pat. No. 4,480,162 are incorporated herein by reference.
A micro-mechanical electrostatic relay that can be employed for the purpose of the invention is disclosed in German PS 42 05 029. This relay has at least one armature that is elastically pivotably connected to a carrier at one side, the free end thereof carrying at least one contact piece and having at least one first electrically conductive layer. Further, the relay has at least one cooperating plate with at least a second electrically conductive layer and at least one cooperating contact piece that interacts with the contact piece of the armature. The two layers lying opposite one another are insulated from each other and can be connected to opposite polarities as of a voltage source. The armature is moved given application of a voltage to the layers acting as electrodes, so that the free end of the armature with the contact piece comes into contact with the cooperating contact piece, whereby the desired electrical connection is produced. The teachings of German PS 42 05 029 are incorporated herein by reference.
The carrier member or the substrate in the electrostatic relay is preferably fabricated of silicon, which exhibits an adequately high insulating resistance despite its semi-conducting properties and is consequently employed as material having an electrically insulating effect in the relay. Silicon has extraordinary good mechanical properties and can be easily micro-structured using known material erosion methods.
Further types of micro-mechanical electrostatic relays are disclosed in German PS 44 37 259 (corresponding to U.S. Pat. No. 5,673,785), German OS 198 54 450, German OS 198 07 214 as well as in German PS 198 23 690. These relays are likewise suitable for employment in a high magnetic field of a magnetic resonance tomography apparatus.
In another preferred embodiment, the electrostatic relay employed in the line system of the magnetic resonance tomography apparatus is constructed without ferromagnetic materials, i.e. its magnetic permeability is especially low. Instead of employing silicon, the substrate or the carrier member and/or a housing-forming cooperating member can be fabricated of a ceramic or of glass, particularly of Pyrex(copyright) glass. The layer electrodes producing the electrostatic attraction are preferably manufactured of gold or of a gold alloy. The non-ferromagnetic structure of the electrostatic relay has the particular advantage for utilization in a magnetic resonance tomograph that a disturbance of the uniform magnetic basic field and a disturbance of the radio-frequency transmission field, that is assumed to be uniform, are avoided. As a result, corresponding image artifacts are suppressed to a significant extent.
In another preferred embodiment, the electrostatic relay is employed to activate or deactivate a radio-frequency coil of the magnetic resonance tomography apparatus by switching the relay.
The radio-frequency coil activated or deactivated in this way is preferably a radio-frequency reception coil that, in particular, is deactivated by the switching before a radio-frequency pulse is transmitted into the subject to be imaged.
For example, the electrostatic relay can be utilized as an interrupt, as a multiplexer or as a demultiplexer or as a part thereof in an electrical high-frequency line leading to the radio-frequency coil.
The switching caused by the electrostatic relay is preferably employed for demultiplexing the reception signals of a number of radio-frequency reception coils. It is likewise preferred that this switching be utilized for multiplexing the control signals for a number of radio-frequency transmission coils.
In a preferred embodiment, the radio-frequency coil is detuned by the switching. This employment achieves a detuning circuit wherein the electrostatic relay serves as an active switch element.
The invention proceeds from the recognition that a conventionally employed detuning circuit as was initially described exhibits a number of disadvantages. In detail, these are:
a) The components of the detuning circuit must have high electrical strength with respect to the adjacent, induced voltage.
b) The components must also be power-resistant because of the high circuit current.
c) The induced current leads to an unwanted local disturbance of the radio-frequency transmission field that is ideally assumed to be homogeneous.
d) In worst case scenarios, the radiated radio-frequency transmission field can interfere at specific points in space with the field that is excited by the current induced in the blocking circuit. Local field elevations resulting therefrom could occur within the patient under examination, so that the risk of an excessively high introduction of eddy currents and, the associated risk of an upper transgression of the locally allowed, specific absorption ratio SAR, would exist.
f) Since the detuning circuit is exactly tuned to the Larmor frequency of the magnetic resonance tomograph, there is a risk of coupling the detuning circuit with other resonant structures that are allocated to the radio-frequency reception coil (for example, further detuning circuits, sheath wave inhibitors) or there is a risk of coupling with other radio-frequency reception coils that are not detuned.
g) Given a fault of a component at the PIN diode and given simultaneous failure of other safety precautions, it could occur that the radio-frequency reception coil is not detuned and that this is not recognized. In this case, melt fuses must be installed into the radio-frequency reception coil circuit as ultimate patient protection. Such fuses, however, have an unwanted negative influence on the image quality due to their finite electrical resistance.
h) The detuning circuit with a PIN diode requires much space, particularly because the inductance (coil) employed therein must have adequate quality as well as adequate current and voltage strength.
The invention is also based on the consideration that electromagnetic relays are not adequately fast for solving the problem initially cited and are thus hardly suitable therefor.
In conjunction with the detuning circuit, the invention is based on the consideration that a switch element suitable for the detuning of the radio-frequency reception coil should be specified by the following properties:
a) It must be suitable for employment in the strong magnetic basic field of the nuclear magnetic resonance tomography apparatus.
b) It must be able to switch adequately fast so that the high repetition rates required for imaging in nuclear magnetic resonance tomography are possible.
c) The switch element must be voltage-stable up to approximately 500 V and above.
d) The resistance in the through-connected state (ON resistance) should be very low, particularly far lower then 1xcexa9, so that the signal-to-noise ratio (S/N) of the radio-frequency reception coil is not negatively influenced.
e) The switch element should guarantee a high service life having at least 108 obtainable switching cycles.
All of these specifications can be met extremely well with an electrostatic relay or an electrostatic switch.
Preferably, a conductor loop is electrically closed or interrupted for activating or for deactivating the radio-frequency coil.
The detuning of the radio-frequency reception coil with the electrostatic relay has a number of significant advantages:
a) Since no current, or no significant current, flows in the detuning circuit in the detuned case, the relay need not necessarily be implemented power-resistant or integrated in a power-resistant manner; rather, it merely has to be voltage stable. As a result, the manufacturability of a detuning circuit is facilitated.
b) Since no significant current flows in the detuning circuit in the detuned state, no heating that is unpleasant or technologically disadvantageous for the patient occurs.
c) Since no current flows in the detuned state, there are also no local distortions in the radio-frequency transmission field that could lead to image artifacts and to a jeopardization of the patient.
d) The detuning circuit is not resonant, so that the risk of coupling with other resonant circuits in the nuclear magnetic resonance tomography apparatus is overcome.
e) As a consequence of the smaller dimensions given a micro-mechanical implementation of the electrostatic relay, the advantage also is achieved that the space needed for the detuning circuit is reduced. This is particularly advantageous when the radio-frequency reception coil is a local reception antenna that the medical personnel must attach to the region under examination in the patient. Due to the reduced space requirement, such a coil can be constructed in a more ergonomic fashion.
The above object directed to a nuclear magnetic resonance tomography apparatus is inventively achieved in a magnetic resonance tomography apparatus of the type initially described wherein the switch element is fashioned as an electrostatic relay.
The relay is preferably a micro-mechanical relay that, in particular, is at least partially manufactured by a material erosion technique, for example by etching, from a substrate, particularly from a silicon substrate.
The advantages cited for the inventive employment of such an electrostatic relay analogously apply to the inventive magnetic resonance tomography apparatus. The same is true of the preferred embodiments of the electrostatic relay that were cited in conjunction with the inventive employment.
In a preferred embodiment, the relay is allocated to the radio-frequency coil such that the radio-frequency coil is deactivated or detuned given an interrupted relay.
The radio-frequency coil can be a radio-frequency transmission coil. Preferably, the radio-frequency coil is a radio-frequency reception coil.
The relay can be connected in an electrical high-frequency line leading to the radio-frequency coil.
Given integration of the relay in the high-frequency line, the relay can be employed to replace the previously employed, resonant sheath wave inhibitors. Instead of a sheath wave inhibitor, two electrostatic relays are utilized that completely separate a coaxial high-frequency cable during the transmission phase.
In a preferred embodiment, the relay is arranged within a conductor loop of the radio-frequency coil such that the conductor loop is interrupted when the relay is opened, i.e. when the relay contacts are open.
In order to increase the dielectric strength, it is expedient to connect at least one further electrostatic relay within the conductor loop, with the two electrostatic relays connected in series.
A number of electrostatic relays connected in parallel are provided for advantageously reducing the volume resistance.
In an electrostatic relay, the layer electrodes that produce the electrostatic attractive force are charged with a constant voltage. A constant voltage source is allocated to the electrostatic relay for this purpose.
Preferably, a drive unit is provided for generating a switching event in the relay, the drive unit having the constant voltage source allocated to it, and the relay being charged with the constant voltage thereof when driven by the drive unit.
A monitoring unit for measuring an electrical current flowing through contact pieces of the relay can be provided. For example, the monitoring unit can charge the relay with a constant current. By measuring this current, it can be recognized whether the relay opens during the transmission phase and thus detunes the radio-frequency reception coil.
Preferably, the monitoring unit generates an error signal when the current exceeds a threshold. Given employment of the electrostatic relay for detuning the radio-frequency reception coil, the error signal is generated only when the measurement cycle in the nuclear magnetic resonance tomography apparatus passes through a radio-frequency transmission phase at the moment. The error signal indicates whether the switch opens during the transmission phase. When this is not the case, the measurement can be aborted. Such an error signal can be so reliably generated that melt fuses as radio-frequency current limiting units, as were hitherto employed for patient protection, can be foregone. The lossy melt fuses degrade the signal-to-noise ratio in the reception signal. Foregoing the melt fuses thus leads to improvement of the signal-to-noise ratio.
An inductor coil is preferably connected in parallel with the electrostatic relay that is employed. As a result, a residual capacitance of the micro-mechanical relay can be compensated and the behavior of the relay given opened relay contacts is thus improved.
For shortening the switching time, it is also advantageous to drive the electrostatic relay with a dynamically regulated DC voltage. An elevated DC voltage is applied for initiating the switching event, this being reduced to a lower value subsequently during a holding phase wherein the switch remains closed.
The above object is also achieved in accordance with the principles of the present invention in a magnetic resonance imaging apparatus having a radio-frequency system with a system status which must be altered during an MR scan, and wherein an electrostatic relay is connected in the RF system, with the electrostatic relay being switched during an MR scan to alter the status of the RF system.
The aforementioned advantages and embodiments related to the relay, as well as those related to the initially described magnetic resonance tomography apparatus, apply as well to this version of a tomography apparatus.